Under the Clean Air Act of 1990, the Environmental Protection Agency (EPA) was given the authority to set the maximum levels of certain pollutants in the air anywhere in the United States. Since these pollutants arose primarily from automotive exhausts, the concept of reformulated gasoline (RFG) was introduced by the EPA in order to help the cities and states with the highest levels of pollution meet the minimum requirements of the National Ambient Air Quality Standards especially with respect to ozone concentration. Both methyl tertiary butyl ether (MTBE) and ethanol were approved as additives to gasoline for this purpose; and domestic refiners have used MTBE for over a decade.
More recently, however, MTBE has been found to be carcinogenic and, even worse, has been found to be leaking from underground storage tanks into groundwater that serves as a source of drinking water. California was the first state to ban MTBE from gasoline; and, since that time, fifteen other states have instituted MTBE bans.
Congress passed the Energy Policy Act of 2005 creating for the first time a Renewable Fuels Standard (RFS) that committed the United States to the use of ethanol to replace MTBE in gasoline and established a baseline for ethanol usage of 4 billion gallons in 2006. While the Energy Policy Act of 2005 did effectively eliminate the 2% oxygen requirement in RFG set by the Clean Air Act, currently, approximately 30% of the gasoline sold in the United States contains ethanol.
Ethanol is currently produced primarily by fermentation of sugars, starches or cellulose in either a batch or continuous process. The mash is heated to eliminate harmful bacteria prior to fermentation. After transfer to a fermentation tank, yeast is added to promote the production of ethanol, which takes 40-50 hours. During fermentation, the tank is agitated either by a mechanical stirrer or by a gaseous air lift. The product of fermentation is a dilute aqueous ethanol stream commonly called “beer” and containing up to 16-18% ethanol by volume. In order to recover the ethanol from “beer,” the liquid (either with or without filtration to remove solids) is fed to a multi-stage distillation column which produces a primary overhead product containing approximately 95 weight percent ethanol. Higher ethanol content cannot economically be achieved by distillation, since ethanol and water form an azeotrope at 96 weight percent ethanol; and the number of distillation trays required to produce this composition would be infinite. As a result, an additional processing step is required involving adsorption by molecular sieve zeolites, which selectively remove water producing a fuel-grade ethanol stream containing greater than about 99 weight percent ethanol.
Both the distillation step to produce 95 weight percent ethanol and the drying of that product using molecular sieves are extremely energy intensive processes resulting in a level of energy required to produce a gallon of ethanol that approaches the energy content of the ethanol produced when burned in gasoline. Therefore, there is a need for an ethanol production process that significantly reduces the energy consumed and results in a much higher “net energy” per gallon of ethanol.